There are currently available on the market various types of adjustable chairs which are particularly designed for office use. One of the most popular structures for an office chair comprises a base which supports a seat and a seat back. Both the seat and the seat back may be provided with independent means for their positional adjustment to suit individual users.
This type of chair structure has substantial disadvantages in that the use of separate mechanisms for the positioning of the seat back and for the positioning of the seat significantly increase the manufacturing costs because of the complexity of the structure. Moreover, in use, such known chair structures have disadvantages due to the fact that, by providing separate actuating mechanisms for the two different adjustments it is difficult to synchronise their positional variation in relation to the positional variation of the human body. In particular, it is not always possible to obtain adequate adaptation of the inclination of the seat back, with consequent lack of convenience and comfort for the utiliser.